without adding all users to the teams, simply edit the "Everybody Else" in Project Settings / Project Team — it sets the default access level for everybody not listed in other teams.

#21

To hide cover images from tickets in Board View...

simply turn them off via Options/View settings.

#22

To show custom ticket fields in Board View...

use the "Custom fields" button (the one with the "eye" icon) on the top of the board.

#23

To add an image directly from your keyboard buffer...

(from example, one taken with Print Screen button on PC) simply use CTRL+V (Command+V on a Mac) when creating or editing a Note. You can also paste images directly into Board View to create tickets with image covers.